1. Field of the Invention
The present invention relates to filter modules that perform input/output of two signals in different frequency bands and to communication apparatuses including such filter modules.
2. Description of the Related Art
Presently, mobile phones use various communication systems, such as GSM and DCS. Accordingly, devices capable of dual-band or triple-band applications that are compatible with a plurality of systems have been developed for mobile phones. In such mobile phones, switching circuits for switching systems and filters for removing signals having frequencies other than operating frequencies.
Such a multi-band filter module which performs input/output of two signals in different frequency bands is disclosed in Japanese Unexamined Patent Application Publication No. 2004-166258.
FIG. 1 illustrates an example of a configuration of a filter module disclosed in Japanese Unexamined Patent Application Publication No. 2004-166258.
In FIG. 1, a first port 100a of a first high-frequency switch 10a is connected to an unbalanced port P1 of the filter module. This high-frequency switch 10a includes three ports. A second port 100b of the first high-frequency switch 10a is connected to an unbalanced port 110a of a first balanced-unbalanced bandpass filter 20a. A third port 100c is connected to an unbalanced terminal 120a of a second balanced-unbalanced bandpass filter 20b. The first and second balanced-unbalanced bandpass filters 20a and 20b are connected to a second high-frequency switch 10b and a third high-frequency switch 10c each including three ports.
A first port 130a of the second high-frequency switch is connected to a first balanced port P2-1 of the filter module. A second port 130b of the second high-frequency switch is connected to a first balanced port 10b of the first balanced-unbalanced bandpass filter 20a. A third port 130c of the second high-frequency switch is connected to a first balanced port 120b of the second balanced-unbalanced bandpass filter 20b. 
A first port 150a of the third high-frequency switch is connected to a second balanced port P2 of the filter module. A second port 150b of the third high-frequency switch is connected to a second balanced port 110c of the first balanced-unbalanced bandpass filter 20a. A third port 150c of the third high-frequency switch is connected to a second balanced port 120c of the second balanced-unbalanced bandpass filter 20b. 
In a circuit disclosed in Japanese Unexamined Patent Application Publication No. 2004-166258, the two filters that are compatible with a plurality of systems and the first to third switches arranged to switch between these systems are used. Thus, communication is enabled by switching to either one of the filters that are arranged to pass signals at desired communication frequencies.
Such a circuit has a problem of being relatively large since it requires the switches 10b and 10c at both the input and output sides.
To address this problem, it may be possible to provide a switch at only one of the input portion and the output portion. However, with such a configuration in which switching is performed using only a switch circuit provided at the input side or output side of the filter, signals that leak from the switch to a filter that is not selected by the switch may be transmitted. Thus, it is likely that signals at frequencies corresponding to the filter that is not selected will be leaked. Therefore, there is a possibility of a communication failure or other malfunction due to the unwanted signals leaking into a currently operating system from another system.